Vacuum rotary drum dryer

Fig. 9.8 A microwave-vacuum rotary drum dryer. Adopted from Kaensupeta/. (2002).

Experimental study on drying of chilli in a combined microwave-vacuum rotary drum dryer. Drying Technol. 20(10) 2067-2079. 

Inorganic Products.—General rules for the application of certain types of dryers cannot be given as there are too many variations in the nature and quantity of the products to be handled. Usually rotary vacuum and drum dryers are used for delicate products, and direct-heated or steam-heated cylindrical rotary dryers are used for bulky materials that are not affected by higher temperatures. Chamber and shelf dryers (either atmospheric or vacuum) should be used for all delicate and sticky materials, also pastes, and in cases where the material is strongly acid or alkaline, as the trays that are placed on top of the shelves can be made of almost any kind of material which will prevent contamination of the product. Sticky material and paste can also be handled successfully on either a single or double roll drum dryer. 

Rotary drum vacuum filters, 11 345, 355-357. See also Drum filters Rotary dryers, 9 121-122 indirect-heat, 9 129 steam-tube, 9 129-130 Rotary fiber spinning process, 13 387 Rotary furnace technologies, 21 391 Rotary hearth furnaces, 13 177—178 Rotary impeller vane meters,... 

At steady state, the product stream is continuously removed from the fermentor at a rate equal to feedrate. The product stream is concentrated in a desludging separator to a paste of 20 to 30% dry solids. After washing with warm water to remove adsorbed or absorbed hydrocarbons, a second desludging separator produces a cleaned paste that is ftirther dehydrated in a rotary vacuum filter. A final drying step in an atmospheric drum dryer reduces the moisture content to less than 10%, and the powdered product is packaged for the customer. 

Conduction or indirect dryers are more appropriate for thin products or for very wet solids. Heat for evaporation is supplied through heated surfaces (stationary or moving) placed within the dryer to support, convey, or confine the solids. The evaporated moisture is carried away by vacuum operation or by a stream of gas that is mainly a carrier of moisture. Vacuum operation is recommended for heat-sensitive solids. Because the enthalpy lost with the drying air in convective dryers is large, their thermal efficiency tends to be low. For conduction dryers the thermal efficiency is higher. Paddle dryers for drying of pastes, rotary dryers with internal steam tubes, and drum dryers for drying thin slurries are examples of indirect dryers. 

The process system consists of two long-tube vertical evaporators, a draft-tube baffled crystallizer, a rotary-drum vacuum filter, and a direct-heat rotary dryer. Also, pumps are needed to move the solution from evaporator 1 to evaporator 2, to recycle the filtrate from the filter to the crystallizer, and to move the magma from the crystallizer to the filter and a heat exchanger is needed to heat the recycle filtrate. However, the purchase costs for the three pumps and the heat exchanger are not considered here because examples for these types of equipment are presented in Section 16.5. For the equipment considered here, assume fabrication from stainless steel, with a material factor of 2 for the ratio of stainless steel cost to carbon steel cost. For the process, using the following size factors and the equations in Table 16.32, the estimated f.o.b. equipment purchase costs at a CE index of 394 are included in the following table. 

Typically, indirect dryers are used for small- or medium-size production. The product from such a unit has a higher bulk density than the same material processed in direct dryers. Particle size degradation usually can be minimized by proper selection of agitator speed or design. The common indirect heated dryers are tubular dryers (with or without vacuum), drum dryers (atmospheric, vacuum, horizontal or rotary vacuum, and others), hollow disk dryers, paddle... 

Another type of dryer where heat conduction is used for drying is vacuum rotary dryers also called a vacuum tumble dryer. Heat is supplied by warm water or any other heat exchange medium circulating through a jacket of rotating-cone drum. Vacuum is applied to be able to dry at lower temperatures and more rapidly. 

Figure 9.4. Types of dryers cited in Tables 9.1 and 9.2. (a) Tray or compartment, (b) Vacuum tray, (c) Vertical agitated batch vacuum drier, (d) Continuous agitated tray vertical turbo, (e) Continuous through circulation, (f) Direct rotary, (g) Indirect rotary, (h) Agitated batch rotary (atmos or vacuum), (i) Horizontal agitated batch vacuum drier, (j) Tumble batch dryer, (k) Splash dryer. (I) Single drum, (m) Spray, (n) Fluidized bed dryer, (o) Pneumatic conveying (mostly after Nonhebel and Moss, 1971).

Potatoes are first cooked, and the pulp is spread by rollers over the surface of large steam heated drying drums. Bituminous coal and all kinds of granular material are handled in tubular dryers. Soap is dried when in pieces in chamber dryers, and soap flakes are handled on ribbon dryers at low temperature. Starch, casein and similar products are handled in tunnel dryers and also in rotary vacuum apparatus. Food products, fruits and vegetables are usually dried by hot air in chamber dryers. Vegetables have also been handled under vacuum, which has given an excellent product. 

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